From WO 2012/104086 A1 discloses a method for locating metallic or metal-containing objects and materials is known, wherein currents in at least two transmission coils are controlled relative to one another such that a reception coil output signal received by at least one reception coil or average values of demodulation phases generated from the reception coil output signal are continuously controlled to “zero” relative to one another, even under metal influence. In connection therewith, by demodulation at least at 0° and by a demodulation offset by 90°, the control values are detected and adapted as values so that a reliable detection of an object to be detected is possible even when other metallic objects are located in the detection region.
The prior later-published German patent application DE 10 2012 001 202 A1 describes a sensor with a coil arrangement wherein the normally circular windings of a coil are configured in a meandering form such that two transmission coils and one reception coil cooperate with one another such that the received signal is cancelled out. Therein a transmission coil is preferably arranged, respectively, above and below the reception coil, These have slightly different rotation angles relative to the reception coil, so that by means of the distribution of the transmission currents, the signal in the reception coil can be regulated to zero. This coil system thus consists of at least two, preferably three independent coil portions. Typically, for reasons of optimum contactability, the windings of the coils begin at the outer edge and are laid inwardly in a meandering form. From there, the current must reach the outside again and this can be achieved by means of a second layer with a meandering coil in which the coil current passes from the inside outwardly again. This increases the structural height.
From WO 2010/133501 A1, there is known an inductive sensor for detecting metallic objects, having a transmission coil arrangement with at least two transmission coils and at least one reception coil. By means of an evaluation and control unit, the transmission coils are energizable with transmission currents, wherein the winding sense of the transmission coils and the sign of the transmission current amplitudes are selected so that both transmission coils generate a magnetic field of the same direction at the location of the reception coil, Transmission coils and reception coils are geometrically arranged such that they overlap one another and induce a potential of zero or almost zero in the reception coil arrangement, i.e. the magnetic fields substantially compensate for one another. Beginning from such a balanced state, the induced voltage is then influenced by an object, resulting in a detection signal. The regulation takes place on the transmission side.
From DE 10 2004 047 189 A1 a metal sensor for building materials is known comprising a transmission coil and a reception conductor loop system which are inductively coupled to one another. The conductor loop system is formed by conductor structures on a circuit board. Since complete compensation does not take place, the measurement signals are digitized and a narrow-band digital filtration is carried out to reduce the noise bandwidth.
From DE 10 2010 043 078 A1, there is known a sensor device as a metal sensor with at least two coils and a magnetic field sensor wherein the arrangement of coils and the magnetic sensor relative to one another and, respectively, the number of coil windings or the winding sense of the coils or the coil current are selected such that the magnetic field generated by the coils approximately disappears at the location of the magnetic field sensor.
From DE 10 2005 002 238 A1, there is known a sensor for locating metallic objects, having a transmission coil and a reception conductor loop system inductively coupled thereto. The transmission coil is connected in series with the primary side of a neutralizing transformer, By this means, the smallest possible offset signal is generated.
For the detection of ferromagnetic substances, from DE 44 42 441 A1, a miniaturized arrangement made using planar technology is known for detecting magnetic permeability. The arrangement comprises coils which generate a magnetic flux, a flux conducting structure and flux-detecting components. Changes in the magnetic permeability are detected by the flux-detecting components, Through production using planar technology, a high degree of miniaturization and thereby a high level of spatial resolution is achievable.
From DE 103 18 350 B3, there is known an inductive proximity switch with a transmission coil and, arranged in its magnetic alternating field, a reception coil, wherein the magnetic flux applied in the reception coil by the alternating magnetic field approaches zero in the switching position or the rest position of the proximity switch. The coils are arranged adjacent and offset to one another, so that the field lines mutually penetrate the coils. One coil is ring-shaped, the other circular, wherein the circular coil overlaps the ring-shaped coil. With this embodiment, the sensing spacing is to the greatest possible extent uncritical in relation to temperature variations and mechanical influences of the coil support body.
From DE 39 12 840 A1, there is known a detector coil arrangement for an inductive detecting device. By the use of printed circuits for the detector coils and the manner of mounting the windings in the form of a conductor band of mutually adjacent conductor tracks on a common support layer, manufacturing and ease of production of the detector coil arrangement are ensured.
From DE 34 39 369 C2. there is known a method for detecting accompanying slag in a stream of metal melts, wherein the transmission coil and the reception coil are embedded in ceramic material. There is applied to the transmission coil a current containing a plurality of frequencies which induce a potential which is evaluated in a frequency-selective manner in the reception coil. The content of slag is determined by means of the pattern of the conductivity distribution, wherein a continuous or partially continuous measurement of the changing temperature of the melt and also of the measuring sensors is carried out.
Inductive proximity sensors are commonly known and widely used, The typical design resembles a section of tube similar to the sensor element 5.1 according to FIG. 5. These sensors typically use a ferrite core for concentrating the field lines of the sensor coil. The installation of a sensor of this type is usually carried out through a hole in a wall which is often made of metal. For this purpose, the corresponding space must also be available. Although sensors without ferrite cores can achieve a great range, the immediate presence of metal, e.g. from a surrounding housing makes these sensors blind for further detection.
Cuboid-shaped sensors for detecting a metallic approach can be screwed onto surfaces. Typically, here also, a ferrite core is used for concentrating the field lines and defining the detection direction. These sensors are therefore relatively thick.